Gasket assemblies are often used as a seal between mating mechanical components. One common application involves gasket assembly placement between the engine block and cylinder head of an internal combustion engine. The engine block and cylinder head are bolted together and the gasket relies on the force of the bolted connection to seal the various openings between the two mating components. In particular, cylinder head gasket assemblies typically extend around cylinder bores to provide a combustion seal, maintaining the high temperature gases of combustion within the combustion opening. Simultaneously, the gasket assemblies also seal fluid flow openings such as coolant and oil openings to prevent undesirable mixing or leakage.
It is typical for a cylinder head gasket assembly to include a main gasket body with a cylinder bore opening, the inner periphery of which is surrounded by a metallic generally U-shaped flange. The flange engages both the upper face and the lower face of the main gasket body. Retained within the flange is a yieldable fire ring typically made from metal. The flange and fire ring work in combination to provide a primary combustion seal while the main gasket body provides structural support for both the flange and the fire ring and acts as a secondary combustion seal.
A yieldable fire ring has drawbacks as a gasket assembly component. It has limited elasticity and tends to plastically deform over time, greatly decreasing its elastic properties. Further, new engines are being introduced which have significantly increased firing pressures and operating temperatures with lower bolt clamping forces and engine component stiffness.
To address some of these concerns main gasket bodies have been formed from composite materials with improved thermal conductivity. However, many of these composite materials have a reduced ability to withstand the stresses and strains induced by the bolted connection of the mating components. In particular, graphite has been the optimum material for use in a variety of composite head gasket designs. It includes improved conformability, heat resistance and relaxation properties when compared to other fiber-based replacement materials. Yet, it has a very low shear strength which allows for the thermally induced lateral relative motion of the cylinder head and engine block to move the flange laterally back and forth. Further, graphite also includes a relatively low spring rate which can allow for greater lift-off deflections at the cylinder bore opening. Unfortunately, the low spring rate remains almost constant even after prolonged exposure to heat. Therefore, the dynamic stresses are unable to decrease over time.
When a flange is used, it relies on both the fire ring and the main gasket body for support. As a result, the flange is subjected to greater dynamic stresses induced through the use of a main gasket body formed from composite materials and may fail over time in a form of combustion seal fatigue failure commonly known as "flange cracking".